In power steering mechanism, engine control mechanism, power transmission device, etc. for vehicle, a necessity for detecting the torque applied to the steering shaft and other passive axes, etc. that are rotating shafts is high.
Generally, concerning a material having the magnetostrictive characteristics, a relative permeability varies when a force is applied thereto, and the relative permeability decreases in a compression force direction while the relative permeability increases in a tensile force direction. A magnetostrictive torque detection apparatus using this principle is disclosed, for example, in prior arts 1 and 2.
The prior art 1 (Japanese Patent Laid-Open No. 1-94230) discloses a magnetostrictive torque detection apparatus shown in FIG. 1A. In this magnetostrictive torque detection apparatus, plural magnetostrictive films 13, 13 are adhered with an angle of ±45° to the rotating shaft 11 on an outer periphery of a rotating shaft 11 that is a passive axis, and ring form detection coils 12, 12 corresponding to the ±45° magnetostrictive films 13, 13 are disposed on an outer periphery thereof. When the torque is applied to the rotating shaft 11 from outside, a compression force occurs in one of the magnetostrictive films 13, 13, while a tensile force occurs in another one, thereby generating a strain, so that the permeability of the magnetostrictive films 13, 13 changes. A change in inductance of the ring form detection coils 12, 12 based on a change in permeability of these magnetostrictive films 13, 13 are detected and output.
The prior art 2 (Japanese Patent Laid-Open No. 6-194239) discloses a magnetostrictive torque detection apparatus shown in FIG. 1B. In this magnetostrictive torque detection apparatus, two strips of detection coils 12, 12 inclined with an angle of ±45° to a magnetostrictive rotating shaft 11 are overlapped and adhered to an inner peripheral surface of a ring form magnetic core 14, to be disposed at a circumference of the magnetostrictive rotating shaft 11, so that the change in permeability of the magnetostrictive rotating shaft 11 is detected and output (FIGS.4 to 6 and Embodiment 4).
FIG. 1C shows another magnetostrictive torque detection apparatus (prior art 3). In this magnetostrictive torque detection apparatus, a change in permeability of a magnetostrictive rotating shaft 11 is detected and output, by using detection coils 12, 12 wound on U-shaped magnetic cores 14, which are disposed to be opposed in regard to the magnetostrictive rotating shaft 11.
However, the magnetostrictive torque detection apparatus according to the prior art 1 has a disadvantage in that it is necessary to process the rotating shaft 11, since the magnetostrictive films 13, 13 with an angle of ±45° are adhered to the rotating shaft 11.
In the magnetostrictive torque detection apparatus according to the prior art 2, it is not necessary to process the rotating shaft 11, however a difference of inductance between two strips of the detection coils 12, 12, which are overlapped and inclined with an angle of ±45° to the inner peripheral surface of the ring form magnetic core 14, is detected, so that it is necessary to detect increase and decrease directions of the inductance for detecting a torque direction. Further, since it is necessary for setting a zero point to detect a condition where inductances of two strips of the detection coils 12, 12 become equal to each other, it is not easy to set the zero point. Still further, since the rotating shaft 11 has a configuration penetrating through the ring form magnetic core 14, it is difficult to install the detection coil 12 thereon.
In addition, in the magnetostrictive torque detection apparatus according to the prior art 3, it is not necessary to process the rotating shaft 11, and it is easy to install on the rotating shaft 11, however, there is a following problem. Since the U-shaped magnetic cores 14 on which the detection coils 12 are wound up are not positioned rotationally symmetrical to the rotating shaft 11 such as a ring form magnetic core, and that there is a part of the rotating shaft 11 which does not face to the U-shaped magnetic cores 14, the zero point of the output signal fluctuates greatly in accordance with rotation of the rotating shaft 11.